IC Cards are generally used in applications in which it is essential for confidential information to be stored and processed securely. By way of example, such applications lie in the fields of health, telephony, pay TV, or banking such as electronic purse applications.
Such cards comprise a plastics card body having an IC or chip device incorporated therein.
In the chip, the integrated circuit forms a complex assembly of logic cells over which a central processor unit (CPU) dispenses and controls information stored in RAM, ROM, or EEPROM type memories by means of a data bus and an address bus.
Conventionally, the logic cells are of the CMOS type. They are constituted by a P-type first MOS transistor and by an N-type second MOS transistor connected in a series and controlled by a common logic control signal resulting from the concomitant action of electrical signals present at the inputs of the circuit and of electrical signals generated by programs contained in the ROM or EEPROM memories or by associated electronic circuits. As a function of the logic control signal, charge distribution between valance and conduction bands is altered, thereby giving rise to controlled switching of said transistors.
Nevertheless, some energy sources can also modify this distribution. This applies in particular to electromagnetic radiation, in particular in ranges going from ultraviolet to infrared. As a result, using such radiation to illuminate a zone of the chip, e.g. a set of logic cells, can cause the transistors in said set of cells to switch independently of any electrical control ordered by the logic circuits.
That is why attackers, by taking a chip connected via its Vdd, Vss, Clock, I/O, and Reset pads, and by illuminating an appropriate zone of its circuits with focused electromagnetic radiation in the ultraviolet, visible, or infrared rant at time t chosen by them, have been able to cause the transistors in said zone to switch, thereby altering the normal sequencing of operations programmed in the memories of the chip, and have in particular been able to cause the chip to perform operations that are normally not authorized, thus giving access to secrets without destroying the circuits.
Known means for protecting integrated circuits against the action of electromagnetic radiation have nevertheless been developed. These comprise software means characterized by the fact that the programs in the ROM or EEPROM memories of the chip are numerous and associated with verification means. Nevertheless, those known means do not provide effective protection against “light” attack and they suffer from the drawbacks of requiring a large amount of memory space in the chip and of significantly slowing down the execution of the operations it is required to perform.